Equipment Fouling

An example of equipment fouling limiting the product RVP occurred in an FCCU debutanizer. The overhead product is a mixture of olefinic and paraffinic C₃ and C₄’s. The bottom product is FCCU gasoline used as a gasoline blendstock.

Additional operating data illustrated how the overhead condensing capacity limited the debutanizer operation. The hot vapor bypass pressure control valve operated nearly closed. Attempts to increase the reflux rate only resulted in the hot vapor bypass control valve closing and the tower pressuring up. The maximum reflux rate of 11.2 MBPSD was 23% below design. The calculated overhead condenser duty was only 23.8 compared to the 29.1 MMBTU/hr design duty. The process outlet was only 111° F compared to the 100° F design even though the actual cooling water supply temperature was 15° F colder than design. The calculated overall heat transfer rate was only 44.3 compared to the 124 design. Obviously, the overhead system was not performing per design.

The lower than design reflux rate raised the butane concentration in the FCCU gasoline from the design value of 0.05 LV% up to 0.7 LV% and greater. The higher butane concentrations resulted in higher FCCU gasoline RVP’s. FCCU gasoline’s large gasoline pool percentage made meeting the refinery’s finished gasoline specifications difficult. Since FCCU capacity was the major refinery limit, the debutanizer overhead condensing limit had a large, detrimental impact upon refinery profitability.

The importance of the operating bottleneck lead to further analysis. All evidence of the poor condenser operation pointed to tube side cooling water fouling. The much lower than design heat transfer coefficient indicated fouling on either the tube or shell sides of the exchanger. The cooling water rate was measured by injecting a known amount of tracer into the exchanger inlet and calculating the water flow rate from the concentration of the tracer in the exchanger outlet. The calculated flow rate of 1,279 gpm cooling water rate was 60% below the 2,292 gpm design flow rate. The cooling water rate was below design despite the fact that the exchangers had 22 psi available pressure drop as compared to only 8 psi in the original design.

The curve comparing the overhead condenser heat transfer with the RVP attainable illustrates how much the overhead condenser fouling affects the bottoms RVP. The base case operation reflecting the survey conditions is shown at the lower left of the curve. The minimum simulation RVP attainable is 7.3 with essentially no C₄- in the bottoms. The other data points on the same line indicate how much the gasoline RVP increases as the overhead condenser heat transfer coefficient is reduced. All of the points below the dashed line denoting the survey U of 44 represent feasible operating points. The top curve estimates the effect of higher cooling water temperatures and hence RVP’s in the summer. Almost all of the points are above the existing U dashed line indicating that the operation is unfeasible with the fouled exchangers. All of the runs maintained the same 1.7 LV% C₅+ in the overhead LPG.

The curve clearly shows how much the survey heat transfer coefficient was below design. The curve for the current winter operation indicates that the RVP could be reduced only marginally by removing all of the C₄-. The curve for summer operation shows the large impact that the poor heat transfer coefficient would have in the coming months. Assuming that the exchanger heat transfer coefficient remained constant, the fouled overhead condensers would result in FCCU gasoline RVP’s above 12. This unacceptably high RVP forced a resolution.

Attempts at online cleaning were unsuccessful. A shutdown was required to clean the bundles. To minimize the expensive downtime, new bundles were bought in advance. The debutanizer was shutdown, and the new bundles were installed. The existing bundles indicated severe tubeside fouling as was expected. Upon startup, the condenser operation returned to design conditions, and the debutanizer no longer limited unit and refinery operation. Lower FCCU gasoline RVP’s helped meet the mandated pool specifications.

 

Minor Equipment Debottlenecking

 

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